For interrupting and making electrical circuits, use is made both of mechanical contacts, sliding contacts such as in the case of current collectors on rail vehicles or commutators in electric motors, fusible links, and of semiconductor switches such as transistors, thyristors and semiconductor relays.
During interruption of the electrical circuits, all these switching elements are exposed to a high self-induced voltage as a result of the rapid reduction of the energy stored inductively in the entire electrical circuit.
Said self-induced voltage heats and destroys semiconductor switching elements and protective circuits, causes material migration and welding at contact areas and can prevent the breaking of the electrical circuit as a result of arcing between contact areas.
During making of an electrical circuit, the capacitance present in the circuit has to be charged rapidly, which momentarily leads to a large switch-on current.
Said switch-on current brings about material migrations at contact areas that have not yet completely closed, and can destroy semiconductor switches through local thermal overloading.
During the transition of the switching elements from the conducting to the blocking state and from the blocking to the conducting state, a power loss is produced at the switching elements due to the simultaneous presence of current and voltage, said power loss being referred to as switching power.
In the case of frequent switching operations, this switching power leads to the heating of the switching elements and of adjacent components, and thereby jeopardizes the reliable operation of entire apparatuses and installation.
In order to protect the switching elements from the harmful effects of the self-induced voltage, use is made of RC circuits, but the latter are heated greatly in the event of high switching frequency.
Diode circuits, also known as freewheeling diodes, protect the switching elements from self-induced voltage only after a response time, but cannot be used with AC voltage, and cause a power loss during each switching operation, which limits the efficiency of frequently switching circuit arrangements such as voltage converters or switched-mode power supplies and leads to the heating and damage thereof.
Furthermore, varistor circuits are known, which protect the switch from particularly high self-induced voltages. However, said varistors are rapidly heated and are therefore unsuitable in the event of high switching frequency and high voltage and also for precise limiting of the overvoltages to low values, for the protection of semiconductor components.
It is also known that the self-induced voltage and heating of the switching element during interruption of the electrical circuit can be effectively limited by means of capacitor connected in parallel with the load or else in parallel with the switching element. However, this circuit has the disadvantage that, during the closing of the switching element, the capacitor would have to be short-circuited or abruptly charged, which causes very high switch-on currents, high switching losses and severe wear of the switching elements, so that the capacitance of the capacitor remains limited to a very small value and the effectiveness thereof is thus greatly restricted.
Taking this as a departure point, it is an object of the invention to specify a circuit arrangement which enables the reliable switching of electrical circuits.